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Pyrazole formation: Examination of kinetics, substituent effects, and mechanistic pathways
Author(s) -
Sloop Joseph C.,
Lechner Brent,
Washington Gary,
Bumgardner Carl L.,
Loehle W. David,
Creasy William
Publication year - 2008
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.20316
Subject(s) - chemistry , pyrazole , substituent , kinetics , regioselectivity , medicinal chemistry , condensation reaction , diketone , reaction rate , reaction mechanism , hammett equation , chemical kinetics , computational chemistry , organic chemistry , reaction rate constant , catalysis , physics , quantum mechanics
Reaction kinetics for the condensation of 1,3‐diketones 1a–o with selected arylhydrazines (aryl = Ph, 4‐NO 2 Ph, 4‐CH 3 OPh, and 2,4‐diNO 2 Ph) was studied using 19 F NMR spectroscopy. Product regioselectivity is modulated by reactant ratios, substituents, and acidity. Reaction rates were found to be influenced by substituents on the diketones and on phenylhydrazines as well as by acidity of the reaction medium with rates varying as much as 1000‐fold. Hammett ρ values for these cyclizations were determined. The reaction was found to be first order in both the diketone and arylhydrazine. The rate‐determining step for pyrazole formation shifts as a function of pH. Mechanistic details and reaction pathways supporting these findings are proposed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 370–383, 2008